1. Field of the Invention
The present invention relates to a spindle motor having a hydrodynamic pressure bearing, and more particularly to a spindle motor that is capable of circulating fluid supplied into dynamic pressure grooves to increase the service life of the fluid and the service life of the motor, and of reducing vertical variation between the center of gravity of a rotary body and the center of dynamic pressure to minimize occurrence of vibration from the motor and improve driving characteristics.
2. Description of the Related Art
Generally, a motor having a ball bearing has a problem in that friction occurs between a sleeve of the ball bearing and a shaft of the ball bearing, by which noise and vibration are generated. Such vibration is called non-repeatable run out (NRRO), which is an obstacle to increasing track density of a hard disk.
On the other hand, a spindle motor having a hydrodynamic pressure bearing maintains the axial rigidity of a shaft of the bearing only using dynamic pressure of lubricating oil due to centrifugal force. As a result, no metal friction of the spindle motor occurs, and the stability of the spindle motor is increased as the spindle motor is rotated at higher speed. Consequently, the spindle motor having the hydrodynamic pressure bearing has the effect of minimizing occurrence of noise and vibration. In the spindle motor having the hydrodynamic pressure bearing, the high-speed rotation of a rotary body is more smoothly carried out than the motor having the ball bearing. As a result, the spindle motor having the hydrodynamic pressure bearing is principally applied to high-end optical disk apparatuses, magnetic disk apparatuses, and hard disk apparatuses.
The hydrodynamic pressure bearing mounted in the spindle motor having the above-mentioned characteristics comprises: a shaft, which is the center of rotation; and a metal sleeve fitted on the shaft such that the metal sleeve and the shaft together define a sliding surface therebetween. At the shaft or the metal sleeve are formed herringbone-shaped or spiral dynamic pressure generation grooves.
In the gap minutely formed at the sliding surface defined between the shaft and the sleeve is filled fluid, for example, lubricating oil such that frictional members are kept not in contact with each other due to hydrodynamic pressure generated from the dynamic pressure generation grooves of the sliding surface. In this way, the hydrodynamic pressure bearing reduces the frictional load when the spindle motor is rotated and supports a rotary member, i.e., a rotor, of the spindle motor.
When the hydrodynamic pressure bearing with the above-stated construction is applied to the spindle motor, the amount of noise generated from the motor is small as rotation of the rotor is supported by the fluid, the power consumption is low, and the impact resistance is excellent.
FIG. 10 is a cross-sectional view illustrating a conventional spindle motor 1 having a hydrodynamic pressure bearing. As shown in FIG. 10, the conventional spindle motor 1 comprises a stator 10 and a rotor 20. The stator 10 comprises: a base 12, in the center of which a metal cylindrical sleeve 32 is disposed; and a plurality of cores 14 disposed on the base 12 while extending in the radial direction thereof about a pole. On at least one of the cores 14 is wound a coil 16.
The rotor 20, which is rotated relative to the stator 10, includes a cup-shaped hub 24. The hub 24 comprises: a boss part 21, in which the upper end of a shaft 34, which is the center of rotation, is fitted; and a skirt part 22, to which a magnet 23 is mounted while the magnet 23 corresponds to the coil 16. The lower part of the shaft 34 is fitted in the sleeve 32.
The sleeve 32 is a rotation-supporting member that is fixedly inserted in a fixing hole 12a formed through the center of the base 12. In the sleeve are formed large and small inner diameter parts 32a and 32b, in which the shaft 34 is fitted. The shaft 34 has large and small outer diameter parts 34a and 34b, in which the large and small inner diameter parts 32a and 32b of the sleeve 34 are fitted, respectively.
At the upper end of the sleeve 32 is disposed a ring-shaped stopper ring 35, which pushes the shaft 34 downward for preventing the shaft 34 from separating from the sleeve 32. At the large and small outer diameter parts 34a and 34b of the shaft 34, which are in contact with the sleeve 32 and the stopper ring 35, are formed dynamic pressure generation grooves G1, G2, and G3, respectively, by which minute gaps, i.e., sliding surfaces, are formed.
When fluid, for example, lubricating oil, is filled in the sliding surfaces defined between the inner diameter of the sleeve 32 and the outer diameter of the shaft 34, an upper thrust dynamic pressure part for generating dynamic pressure according to relative rotation is formed between the lower surface of the stopper ring 35 and the upper surface of the large outer diameter part 34a of the shaft 34, and a lower thrust dynamic pressure part for generating dynamic pressure according to relative rotation is formed between the lower surface of the large outer diameter part 34a of the shaft 34 and the bottom surface of the large inner diameter part 32a of the sleeve 32.
Between the inner circumferential surface of the small inner diameter part 32b of the sleeve 32 and the outer circumferential surface of the small outer diameter part 34b of the shaft 34 is also formed a radial dynamic pressure part for generating dynamic pressure according to relative rotation.
When the conventional spindle motor 1 with the above-stated construction is operated, however, the center of gravity C1 of the rotary body, including the shaft 34 and the rotor 20, is formed in the vicinity of the upper end of the shaft 34 fitted in a connection hole 24a formed at the center of the hub 24 while the center of radial dynamic pressure C2 of the radial dynamic pressure part formed between the small outer diameter part 34b of the shaft 34 and the small inner diameter part 32b of the sleeve 32 is formed at the lower part of the shaft 34, i.e., at the middle of the small outer diameter part 34b of the shaft 34.
As a result, the position where the center of gravity C1 of the rotor 20 is formed from the shaft 34 is relatively away from the position where the center of radial dynamic pressure C2 of the radial dynamic pressure part formed at the shaft 34, as shown in FIG. 10, and therefore, vertical variation T occurs between the center of gravity C1 and the center of radial dynamic pressure C2.
When the spindle motor 1 is operated in the above-mentioned state, and therefore, the rotor is rotated in one direction, the rotary body, including the rotor 20 and the shaft 34, is eccentric due to the vertical variation T. As a result, the vibration characteristics of the spindle motor are deteriorated. As the vertical variation T between the center of gravity C1 and the center of radial dynamic pressure C2 is increased, the rate at which vibration is generated is also increased.
At the inner circumferential surface of the stopper ring 35 is formed a tapered surface 35a to provide an oil storing part for storing a predetermined amount of oil between the tapered surface 35a of the stopper ring 35 and the outer diameter part of the shaft 34. However, the position of the oil storing part is high, and the length of a passage connected between the oil storing part and the outside is small. Consequently, when the spindle motor is operated, oil discharged from the oil storing part easily leaks to the outside through the passage between the hub 24 and the stator 10.
As a result, the stator 10 is contaminated by the oil leaking to the outside, and a rotary object, i.e., a medium, rotating along with the rotor 20 is also contaminated by the leaking oil.
The lower end of the shaft 34, the upper end of which is fixedly fitted in the hub 24 of the rotor 20, extends to the bottom surface of the inner diameter part of the sleeve 32. Also, the large outer diameter part 34a of the shaft 34 extends in the outer circumferential direction. The shaft 34 having the above-stated shape is rotated along with the hub 24. As a result, the total weight of the rotor 20 is increased, which increases inertia of the rotor 20 when the spindle motor is operated, and therefore, the accurate control of the speed of the spindle motor is difficult, and the impact resistance is lowered.
Furthermore, it is necessary that the thrust dynamic pressure generation grooves G1 and G2 be provided to form the upper thrust dynamic pressure part between the lower surface of the stopper ring 35 and the upper surface of the large outer diameter part 34a of the shaft 34 and the lower thrust dynamic pressure part between the lower surface of the large outer diameter part 34a of the shaft 34 and the bottom surface of the large inner diameter part 32a of the sleeve 32. Also, it is necessary that the radial dynamic pressure generation groove G3 be provided to form the radial dynamic pressure part between the inner circumferential surface of the small inner diameter part 32b of the sleeve 32 and the outer circumferential surface of the small outer diameter part 34b of the shaft 34.
However, it is necessary that the dynamic pressure generation grooves G1, G2, and G3 be precisely formed at the shaft 34 with a precision of the μm level. As a result, the costs necessary to precisely form the dynamic pressure generation grooves G1, G2, and G3 are increased, and therefore, the manufacturing costs of the spindle motor are also increased.
In addition, it is difficult to reduce the length of the shaft 34, when the spindle motor is designed, because the radial dynamic pressure generation groove G3 necessary to form the radial dynamic pressure part is provided at the shaft 34. As a result, it is not possible to reduce the height of the spindle motor, and therefore, the miniaturization of the spindle motor is limited.